Reproduction steps:

1. Create a new project.
2. Select Metal graphics API for the Editor.
2. Create a terrain.
3. Focus it, so most of it is visible.
4. Select a terrain.
5. Select Raise/Lower terrain tool.
6. Start painting wildly(Experiment with different brush types).
- squares appears randomly on a terrain surface(Picture attached).

Reproduced in:
5.6.0a4, 5.6.0a5